The work to be done will include: 1) The continuation of development of an in vitro system which responds to the in vitro addition of vitamin K and which carries out the in vitro carboxylation of the protein precursor in the liver to produce prothrombin. 2) The isolation in pure form of the carboxylating enzyme from rat liver. 3) The isolation of the functional form (derivative) of vitamin K. 4) The determination of the relationship between the functional form of vitamin K and the carboxylating enzyme. 5) The elucidation of the mechanism whereby a vitamin K derivative participates in the activation of CO2 and in its transfer to precursor protein to yield prothrombin.